In the area of human computer interfaces there is the need to measure the position of the hand relative to a computer monitor. Various sensing technologies have been used to measure the position including electromagnetic (E/M), ultrasonic, infrared and mechanical linkages. Electromagnetic position sensing technology typically consists of an electromagnetic field generating transmitter and an electromagnetic field sensor. Electromagnetic position sensing technology provides the advantage that there need be no rigid physical connection between the hand and the monitor. Electromagnetic technology, however, possesses the disadvantage that it is sensitive to the presence of metallic objects and other electromagnetic interference generating sources. The electromagnetic fields generated by the electromagnetic transmitters typically adversely affect monitor image quality. Another disadvantage of E/M technology is that it typically includes lag time which renders the position data non-real time. Nevertheless, the convenience of not having bulky mechanical mechanisms attached to the body have encouraged the use of electromagnetic technology, despite its many shortcomings.
For example, the Polhemus system addresses freedom of motion requirements quite well using a tiny electromagnetic sensing element. Unfortunately, the Polhemus system depends upon a desk top transmitter whose signal is extremely sensitive to disturbance by common metal objects such as pens, keys, watches, and the like and whose signal strength falls off approximately exponentially with distance. Thus, the Polhemus system may produce noisy, uncertain data that deteriorates rapidly with distance. Additionally, the Polhemus sensing technology is understood to be too slow for virtual reality applications that aim to be convincing and transparent to the user. While the human eye can perceive stationary images flashed at rates of 30 Hz and faster as continuous movement, moderately quick limb movements can leave a 30 Hz sensing system several inches behind causing a noticeable and disorienting lag on the virtual reality display.
As with E/M position sensing technology, ultrasonic (US) and infrared (IR) position sensing technologies do not require a direct tether between the hand and monitor. US and IR technologies have the disadvantage, however, that they both require direct line of sight. Thus, when one hand passes in front of the other, the position signal can be lost. Additionally, US technology, in particular, is very sensitive to ambient acoustic noise. Both technologies can introduce lag time, again rendering the position data non-real time.
There is therefore a need for a position sensing or tracking device which is accurate, insensitive to environmental influences, has little lag time and has high data rates. Mechanical position sensing devices are desirable, which do not have the shortcomings of the other devices, while allowing for free movement without interference.